NOTE : this guidance pertains to mandatory elements ( e.g. min = 1) if not mandatory  then omit the element from the resource The request to add the text: if the value set does not have the appropriate “unknown” concept code, use  unknown  from the  DataAbsentReason Code System . to the required binding section is not persuasive because it is not conformant to the definition of a required binding: required To be conformant, the concept in this element SHALL be from the specified value set.   In addition to the text for required bindings that is already present - see attached. Will add example snippet add additional guidance:  "if the value set does not have the appropriate “unknown” concept code you must use a concept from the value set otherwise the instance will not be conformant" will update the guidance for required binding error reporting to:  If one of these a status code is missing, a  404  http error code and an OperationOutcome  SHALL  be returned in response to a read on the resource or if returning a response to a search, the problematic resource SHALL be excluded from the search set and and 'warning' OperationOutcome SHOULD be included indicating that additional search results were found but could not be compliantly expressed and have been suppressed".